The present invention relates to screws and the formation thereof; and it relates to dies for forming self-thredding lobular screws.
In one particular, the present invention relates to vibration-resistant screws.
It is, of course, well known to design screws in such a way that they will resist vibrations and remain in the threaded bores into which they are turned. However, problems are encountered because such screws have shank portions which yieldably flex to a substantial degree because of the spring, yieldable nature of the structure. Because of this latter property, from which the characteristic of resistance to vibrations flows, it is difficult to achieve from such screws the additional capacity for efficiently forming a thread so that the screws can be considered as self-tapping as well as vibration-resistant.
On the other hand, self-tapping screws which form their own threads as they are turned into a bore have relatively rigid shanks and threads so that the threads and the remainder of the shank will be able to withstand the excessive torque to which the screw is subjected during the thread-forming operation.
It has already been proposed to provide a vibration-resistant screw structure which is also self-tapping by giving the shank a lobular configuration. However, at the present time, screws of this latter type are still sufficiently springy to yield undesirably under stress. Moreover, such screws are expensive to manufacture, requiring a plurality of operations in the manufacturing process.
A further difficulty is encountered in the formation of self-tapping screws having lobular thread formations in that certain lobular configurations do not lend themselves readily to formation by rolling between a pair of dies which move along in fixed parallel planes.